Adult flatworms parasitize blood capillaries of either the mesenteries or plexus of the bladder, depending on the infecting species. They are unique among trematodes and any other flatworms in that they are dioecious with distinct sexual dimorphism between male and female. Thousands of eggs are released and reach either the bladder or the intestine (according to the infecting species), and these are then excreted in urine or feces to fresh water. Larvae must then pass through an intermediate snail host, before the next larval stage of the parasite emerges that can infect a new mammalian host by directly penetrating the skin.

Estimated speciation dates for the japonicum group: ~3.8 million years ago for S. japonicum/South East Asian schistosoma and ~2.5 million years ago for S. malayensis/S. mekongi.

Schistosoma turkestanicum is found infecting red deer in Hungary. These strains appear to have diverged from those found in China and Iran.[8] The date of divergence appears to be 270,000 years before present.

Taxonomy

The genusSchistosoma as currently defined is paraphyletic, so revisions are likely. Over twenty species are recognised within this genus.

The genus has been divided into four groups — indicum, japonicum, haematobium and mansoni. The affinities of the remaining species are still being clarified.

Thirteen species are found in Africa. Twelve of these are divided into two groups — those with a lateral spine on the egg (mansoni group) and those with a terminal spine (haematobium group).

S. leiperi and S. matthei appear to be related.[9]S. margrebowiei is basal in this group.[10]S. guineensis is the sister species to the S. bovis and S. curassoni grouping. S. intercalatum may actually be a species complex of at least two species.[11][12]

S. spindale is widely distributed in Asia, but is also found in Africa.

S. incognitum appears to be basal in this genus. It may be more closely related to the African/Indian species than to the Southeast Asian group. This species uses pulmonate snails as hosts.

New species

Four additional species have been transferred to this genus.[7] These were previously classified as species in the genus Orientobilharzia. Orientobilharzia differs from Schistosoma morphologically only on the basis of the number of testes. A review of the morphological and molecular data has shown that the differences between these genera are too small to justify their separation. The four species that have been transferred to this genus are

Schistosoma bomfordi

Schistosoma datta

Schistosoma harinasutai

Schistosoma turkestanicum

Examination of the mitochondria suggests that Schistosoma incognitum may be a species complex.[14]

S. haematobium, commonly referred to as the bladder fluke, originally found in Africa, the Near East, and the Mediterranean basin, was introduced into India during World War II. Freshwater snails of the Bulinus genus are an important intermediate host for this parasite. Among final hosts humans are most important. Other final hosts are rarely baboons and monkeys.[17]

S. intercalatum. The usual final hosts are humans. Other animals can be infected experimentally.[17]

S. japonicum, whose common name is simply blood fluke, is widespread in East Asia and the southwestern Pacific region. In Taiwan this species only affects animals, not humans. Freshwater snails of the Oncomelania genus are an important intermediate host for S. japonicum. Final hosts are humans and other mammals including cats, dogs, goats, horses, pigs, rats and water buffalo.[17]

S. malayensis This species appears to be a rare infection in humans and is considered to be a zoonosis. The natural vertebrate host is von Muller's rat (Rattus muelleri). The snail host(s) are Robertsiella species (R. gismanni, R. kaporensis and R. silvicola (see Attwood et al. 2005 Journal of Molluscan Studies Volume 71, Issue 4 pp. 379–391).

S. mekongi is related to S. japonicum and affects both the superior and inferior mesenteric veins. S. mekongi differs in that it has smaller eggs, a different intermediate host (Neotricula aperta) and longer prepatent period in the mammalian host. Final hosts are humans and dogs.[17] The snail Tricula aperta can also be experimentally infected with this species.

Reproduction

Unlike other trematodes, the schistosomes are dioecious, i.e., the sexes are separate. The two sexes display a strong degree of sexual dimorphism, and the male is considerably larger than the female. The male surrounds the female and encloses her within his gynacophoric canal for the entire adult lives of the worms, where they reproduce sexually.

Genome

History

The eggs of these parasites were first seen by Theodor Maximilian Bilharz, a Germanpathologist working in Egypt in 1851 who found the eggs of Schistosoma haematobium during the course of a post mortem. He wrote two letters to his former teacher von Siebold in May and August 1851 describing his findings. Von Siebold wrote a paper (published in 1852) summarizing Bilharz's findings. Bilharz wrote a paper in 1856 describing the worms more fully and he named them Distoma haematobium. Their unusual morphology meant that they could not be comfortably included in Distoma. So in 1856 Meckel von Helmsback created the genusBilharzia for them. In 1858 Weinland proposed the name Schistosoma (Greek: "split body") after the male worms' morphology. Despite Bilharzia having precedence, the genus name Schistosoma was officially adopted by the International Commission on Zoological Nomenclature. The term Bilharzia to describe infection with these parasites is still in use in medical circles.

In 1898, all the then known species were placed in a subfamily by Stiles and Hassel. This was then elevated to family status by Looss in 1899. Poche in 1907 corrected a grammatical error in the family name. The life cycle was determined by the Brazilian parasitologist Pirajá da Silva (1873-1961) in 1908.

In 2009, the genomes of Schistosoma mansoni and Schistosoma japonicum were decoded [18][19] opening the way for new targeted treatments. In particular, the study discovered that the genome of S. mansoni contained 11,809 genes, including many that produce enzymes for breaking down proteins, enabling the parasite to bore through tissue. Also, S. mansoni does not have an enzyme to make certain fats, so it must rely on its host to produce these.[22]